Nice but the diff's fail contrast levels for accessibility.
I have trouble with contrast and I can barely read the diffs and certainly wouldn't use it for any serious amount of time.
Do you have examples of a color scheme which would work better, or other suggestions on how we could improve it? I'm thinking the ability to customize color schemes would be a useful feature to add.
Not really actually, I was going to recommend Solarized[1] and base16[2] but both fail to pass AAA according to WAVE[3]. I'd recommend installing the WAVE extension for Chrome or Firefox as a start point.